Javalin - A simple web framework for Java and Kotlin

Javalin is a very lightweight web framework for Kotlin and Java which supports WebSockets, HTTP2 and async requests. Javalin’s main goals are simplicity, a great developer experience, and first class interoperability between Kotlin and Java.

Javalin is more of a library than a framework. Some key points:

• You don't need to extend anything
• There are no @Annotations
• There is no reflection
• There is no other magic; just code.

General information:

• The project webpage is javalin.io.
• Documentation: javalin.io/documentation
• Chat: https://gitter.im/javalin-io/general
• Contributions are very welcome: CONTRIBUTING.md

Quickstart

Add dependency

<dependency>
    <groupId>io.javalin</groupId>
    <artifactId>javalin</artifactId>
    <version>3.1.0</version>
</dependency>

// or gradle, if you must
compile 'io.javalin:javalin:3.1.0'

Start programming (Java)

import io.javalin.Javalin;

public class HelloWorld {
    public static void main(String[] args) {
        Javalin app = Javalin.create().start(7000);
        app.get("/", ctx -> ctx.result("Hello World"));
    }
}

Start programming (Kotlin)

import io.javalin.Javalin

fun main() {
    val app = Javalin.create().start(7000)
    app.get("/") { ctx -> ctx.result("Hello World") }
}

Examples

This section contains a few examples, mostly just extracted from the docs. All examples are in Kotlin, but you can find them in Java in the documentation (it's just syntax changes).

Api structure and server config

val app = Javalin.create { config ->
    config.defaultContentType = "application/json"
    config.autogenerateEtags = true
    config.addStaticFiles("/public")
    config.asyncRequestTimeout = 10_000L
    config.dynamicGzip = true
    config.enforceSsl = true
}.routes {
    path("users") {
        get(UserController::getAll)
        post(UserController::create)
        path(":user-id") {
            get(UserController::getOne)
            patch(UserController::update)
            delete(UserController::delete)
        }
        ws("events", userController::webSocketEvents)
    }
}.start(port)

WebSockets

app.ws("/websocket/:path") { ws ->
    ws.onConnect { ctx -> println("Connected") }
    ws.onMessage { ctx ->
        val user = ctx.message<User>(); // convert from json string to object
        ctx.send(user); // convert to json string and send back
    }
    ws.onClose { ctx -> println("Closed") }
    ws.onError { ctx -> println("Errored") }
}

Filters and Mappers

app.before("/some-path/*") { ctx ->  ... } // runs before requests to /some-path/*
app.before { ctx -> ... } // runs before all requests
app.after { ctx -> ... } // runs after all requests
app.exception(Exception.class) { e, ctx -> ... } // runs if uncaught Exception
app.error(404) { ctx -> ... } // runs if status is 404 (after all other handlers)

app.wsBefore("/some-path/*") { ws ->  ... } // runs before ws events on /some-path/*
app.wsBefore { ws -> ... } // runs before all ws events
app.wsAfter { ws -> ... } // runs after all ws events
app.wsException(Exception.class) { e, ctx -> ... } // runs if uncaught Exception in ws handler

JSON-mapping

var todos = arrayOf(...)
app.get("/todos") { ctx -> // map array of Todos to json-string
    ctx.json(todos)
}
app.put("/todos") { ctx -> // map request-body (json) to array of Todos
    todos = ctx.body<Array<Todo>>()
    ctx.status(204)
}

File uploads

app.post("/upload") { ctx ->
    ctx.uploadedFiles("files").forEach { (contentType, content, name, extension) ->
        FileUtil.streamToFile(content, "upload/$name")
    }
}

OpenAPI (Swagger)

Javalin has an OpenAPI (Swagger) plugin. Documentation can be enabled both through a DSL and through annotations, and Javalin can render docs using both SwaggerUI and ReDoc. Read more at https://javalin.io/plugins/openapi.

Special thanks

• Blake Mizerany, for creating Sinatra
• Per Wendel, for creating Spark
• Christian Rasmussen, for being a great guy
• Per Kristian Kummermo, also for being a great guy